The present invention relates to a technique of evaluating a processing effect of various processes using laser light. The invention also relates to a technique of relatively evaluating and controlling illumination energy of laser light.
xe2x80x9cLow-temperature processesxe2x80x9d are now being developed to manufacture a liquid crystal panel using polysilicon thin-film transistors (TFTs). This is intended to suppress the cost of a liquid crystal panel itself by using a low-temperature process, which allows use of a glass substrate with which a large-size substrate can be obtained at a low cost.
To realize a low-temperature process, the key subject is to crystallize an amorphous silicon film formed on a glass substrate by a heating process of less than about 600xc2x0 C., a temperature range in which the glass substrate can endure. There is known a low-temperature process in which an amorphous silicon film is formed on a glass substrate by CVD and converted to a crystalline silicon film by illumination with excimer laser light. In this process, the amorphous silicon film is crystallized by instantaneously rendering the surface and its vicinity of the amorphous silicon film into a molten state.
A crystalline silicon film that has been crystallized by illumination with laser light, particularly excimer laser light, is advantageous in that it is close and has superior electrical characteristics. Further, a substrate receives very little thermal damage. However, the excimer laser light is associated with a problem that its illumination energy is unstable, resulting in a difficulty in keeping the optimum illuminating condition.
In the excimer laser, a particular gas is excited by subjecting high-frequency discharging to it, and electromagnetic waves are utilized, which are emitted when molecules of the gas transfer from the excited state to the steady state. Therefore, there exists a problem originating from the principle that when laser oscillation is continued, increase of impurities in the gas or change in quality of the gas itself lowers the laser light output even with application of the same discharge power. It is a general procedure to obtain a constant laser light output by using a calibration table or the like. But this is not always satisfactory. (For example, the illumination energy of laser light is greatly varied by contamination or the like in a discharge chamber.)
It has been proved that the characteristics of a thin-film transistor produced by using a crystalline silicon film that has been crystallized by illumination with laser light approximately depend on the illumination energy of laser light. Therefore, if the illumination energy of laser light can be made constant or a desired value, a thin-film transistor having intended characteristics can be obtained. This is not limited to the thin-film transistor, but also is widely applicable to other semiconductor devices that are produced by a process including laser light illumination.
There are several methods of evaluating the annealing effects of laser light illumination on a semiconductor. Examples of these techniques are disclosed in Japanese Unexamined Patent Publication Nos. Sho. 58-15943, Sho. 58-40331, and Hei. 1-16378.
In these methods, prescribed anneal effects of laser light illumination on a semiconductor, particularly its crystallinity, are measured by Raman spectroscopy, to evaluate the annealing effects. However, the Raman spectroscopy has the following problems.
(1) Bad reproducibility of measurements.
(2) Use of a large-output laser such as an Ar laser causes a problem in safety.
(3) An expensive apparatus is needed.
(4) A measurement takes long time.
It is difficult to evaluate the flatness of a film surface by the Raman spectroscopy, through the flatness of a film surface is an important factor of determining the characteristics of a thin-film transistor manufactured. Thus, a crystalline silicon film to be used for a thin-film transistor having desired characteristics cannot be evaluated sufficiently only by the Raman spectroscopy.
In the above circumstances, at present, in addition to the above-described evaluation using the Raman spectroscopy, the flatness of a film is evaluated by human eyes using an optical microscope or a SEM (scanning electron microscope).
As described above, at present, a crystalline silicon film is produced in the following manner, and a thin-film transistor having desired characteristics is formed by using the crystalline silicon film thus produced.
(A) In a process of crystallizing an amorphous silicon film by using excimer laser light, the optimum illumination condition of excimer laser light is found experimentally. And it is tried to always perform laser light illumination under the optimum condition.
(B) The optimum condition is set by evaluating the crystallinity of the film by the Raman spectroscopy and evaluating its flatness by visual observation.
However, as described above, the illumination energy of excimer laser light is liable to vary and it is difficult to control the illumination energy. As mentioned above as item (B), the evaluation of the effects of laser light illumination depends on the two parameters of the crystallinity evaluation by the Raman spectroscopy and the evaluation of the film flatness by visual observation. It is therefore difficult to control, using the two parameters, the illumination power of excimer laser light, which tends to gradually change, so that it is kept at the optimum value.
The present invention is intended to attain at least one of the following objects.
(1) To provide a technique capable of judging, on a realtime basis, the effects of various processes using laser light, such as a process of improving the quality of a thin film and annealing of a thin film.
(2) To provide a technique capable of performing laser light illumination while making control for always maintaining the optimum condition in a process of improving the quality of a thin film and annealing of a thin film both using laser light.
(3) To provide a technique capable of easily evaluating the crystallinity of a silicon thin film in a crystallization process of a silicon thin film using laser light.
(4) To provide a technique capable of easily evaluating the crystallinity of a silicon thin film and the flatness of its surface in a crystallization process of a silicon thin film using laser light.
(5) To provide a technique of controlling the illumination energy of laser light so that it is always kept close to a predetermined value.
According to one of principal aspects of the invention, there is provided an optical processing method comprising the steps of:
forming a semiconductor thin film on a substrate having an insulative surface;
irradiating laser light or high-intensity light onto the thin film;
measuring a refractive index of the thin film to which the laser light or the high-intensity light has been irradiated; and
controlling an illumination energy of the laser light or the high-intensity light based on the measured refractive index.
In the above method, examples of the substrate having an insulative surface are a glass substrate, a quartz substrate, other various insulative substrates, semiconductor substrates or conductor substrates on which an insulative film is formed, and substrates of other materials on which an insulative film is formed.
Examples of the thin film are an amorphous silicon film and a crystalline silicon film which are semiconductor thin films. The conductivity type of a semiconductor is not limited specifically. Other examples of the thin film are thin films made of an oxide material, a nitride material, a metal material, or an organic material, i.e., a material whose quality is changed by illumination with laser light or high-intensity light.
Examples of laser light are excimer laser light of KrF, ArF or XeCl. High-intensity light may have any necessary wavelength from the ultraviolet range to the infrared range. A laser beam may have any shape suitable for each use, such as a rectangular shape, a linear shape, a point-like shape, or a planar shape.
An example of the method of measuring the refractive index of a thin-film is a method using ellipsometry.
An example of the method of controlling the illumination energy of laser light or high-intensity light is, in the case of excimer laser light, a method of controlling the discharge output.
The above processing method is characterized by evaluating the effects of the laser light illumination by measuring the refractive index of semiconductor thin film whose quality has been changed by the illumination with laser light. For example, a desired effect can always be obtained by controlling the laser light illumination energy so as to always produce a particular refractive index. Or an effect of the laser light illumination can be made within a certain range by causing the refractive index of a laser-light-irradiated semiconductor film to fall within a predetermined range.
According to another principal aspect of the invention, there is provided an optical processing method comprising the steps of:
forming an amorphous silicon film on a substrate having an insulative surface;
crystallizing the amorphous silicon film with the aid of at least one element for facilitating crystallization of the amorphous silicon film;
irradiating laser light or high-intensity light to the crystallized silicon film;
measuring a refractive index of the silicon film to which the laser light or the high-intensity light has been irradiated; and
controlling an irradiation energy of the laser light or the high-intensity light based on the measured refractive index.
The above method is characterized in that the silicon film to be irradiated with laser light is a film that has been crystallized with the aid of at least one element for facilitating crystallization. The at least one element may be one or a plurality of elements selected from Ni, Pd, Pt, Cu, Ag, Au, In, Sn, Pb, As and Sb. In particular, remarkable effects can be obtained when Ni is used. Specifically, an amorphous silicon film can be crystallized to obtain a crystalline silicon film by introducing the element for facilitating crystallization into the amorphous silicon film and the subjecting it to a heat treatment. The heat treatment can be performed at a temperature lower more than 50xc2x0 C. compared with the case where no catalyst element for facilitating crystallization is used. In addition, heating damage to the substrate (particularly a glass substrate) can be greatly reduced.
The at least one elements for facilitating crystallization may be one or a plurality of elements selected from the elements of VIII, IIIb, IVb and Vb families.
According to still another principal aspect of the invention, there is provided an optical processing apparatus comprising:
means for irradiating laser light or high-intensity light to a thin film; and
means for controlling irradiation energy of the laser light or the high-intensity light based on a refractive index of the thin film to which the laser light or the high-intensity light has been irradiated.
In the above processing apparatus, an example of the means for controlling the irradiation energy of laser light or high-intensity light is a mechanism of controlling discharge power of an excimer laser, for example.
In the above processing apparatus, the irradiation energy of laser light or high-intensity light can be made equal or close to a predetermined value, or within a predetermined range by controlling the irradiation energy of laser light or high-intensity light so that the refractive index of the thin film becomes a predetermined value or falls within a predetermined range. Further, by repeating the above operation, the refractive index value can be gradually made close to a predetermined value.
The irradiation energy of laser light can be evaluated in a relative manner by measuring the refractive index of a thin film whose quality has been changed by irradiation with laser light. For example, the irradiation energy of laser light can always be made close to a particular value by adjusting the laser light irradiation energy so as to always produce a constant refractive index. Therefore, even where the irradiation energy of laser light is liable to vary, the variation can be made as small as possible by monitoring the irradiation energy value using the refractive index. In other words, by measuring the refractive index of a thin film whose quality is changed by the irradiation with laser light, a variation of the laser light irradiation energy value can be monitored and the refractive index can be caused to have a predetermined value or fall within a predetermined range. Further, the laser light irradiation energy value can be caused to have a predetermined value or fall within a predetermined range. By utilizing this fact, the effects of the laser light irradiation can be made predetermined ones.
For example, FIG. 4 shows experimental data representing a relationship between the irradiation energy density of laser light and the refractive index n of a silicon thin film whose crystallinity has been improved by the laser light irradiation. Based on this graph, the refractive index of a silicon thin film can be made close to a predetermined value by increasing the laser light irradiation energy density in the next irradiating operation when the refractive index n of a silicon thin film is larger than a predetermined value, and decreasing the laser light irradiation energy density in the next irradiating operation when the refractive index n of a silicon thin film is smaller than the predetermined value.
With the above operation, even where the laser light irradiation energy density is liable to vary, the variation can be recognized from the refractive index of a silicon film whose crystallinity has been improved by the laser light irradiation, so that the laser light irradiation density can be so controlled as to always allow laser light irradiation at a predetermined energy density. Thus, the effects of laser light irradiation can be made constant.